CFM, Velocity & Area Calculator
The continuity equation Q = V × A, solved whichever way you need it — for ducts, grilles, louvers and coil faces.
Solve for
Shape
Enter positive values, then select Calculate.
A designer needs to know the velocity in a 16 in round branch carrying 1,200 CFM — and whether that fits the acoustic target for an open office.
- 1
Find the free area of a 16 in round duct
A = π × (16/12)² / 4 = 1.396 ft²
Diameter in feet, squared, times π/4.
- 2
Solve Q = V × A for velocity
V = Q / A = 1,200 / 1.396 = 859 FPM
Same equation the calculator uses, solved for FPM.
- 3
Check against the velocity guidance
859 FPM is inside typical supply-branch ranges for occupied spaces
The interpretation card flags when a result drifts toward noise or pressure-drop territory.
Takeaway: One equation, three unknowns — enter any two of CFM, FPM and area and the third falls out. For grilles and louvers, always use the manufacturer's free area, not the face size.
Method and assumptions
- Q (CFM) = V (FPM) × A (ft²). Area is the free internal cross-section; round A = πD²/4.
- For grilles, louvers and diffusers use the manufacturer's free (effective) area, not the nominal face size.
- Rectangular sizes from area are geometric only — for friction-equivalent sizing use the Duct Sizer or Rect ↔ Round converter (Huebscher equivalent diameter).
- Airflow and velocity shown to the nearest whole unit, area to 0.001 ft².
This calculator is for preliminary design guidance only. Final duct sizing, construction, and installation must be performed by qualified professionals and comply with applicable codes and standards.